1. Field of the Invention
The present invention relates generally to electrical wiring devices, and particularly to protective wiring devices.
2. Technical Background
Electric circuits are installed throughout a structure such that electrical service is readily accessible to people that live or work in that structure. An electric circuit includes electrical wires that interconnect electrical wiring devices that are positioned at various locations throughout a structure. There are a variety of electrical wiring devices available to the consumer including outlet receptacles, light switches, dimmers, ground fault circuit interrupters, arc fault circuit interrupters and the like.
Ground fault circuit interrupters (GFCIs), transient voltage surge suppressors (TVSS), surge protective devices (SPDs) and arc fault circuit interrupters (AFCIs) are examples of electrical protective devices. They are used to protect persons and structures from being harmed by electrical fault conditions. Protective devices are typically equipped with a set of interrupting contacts that are used to break the connection between the line terminals and load terminals when a fault condition is detected.
With respect to current industry trends, designers of protective electrical wiring devices are faced with two conflicting objectives. The first objective is to reduce the size of the wiring device housing in each of its three dimensions because smaller wiring devices are easier to install in standard wall box openings. At the same time, consumers are demanding that protective devices include additional protective capabilities and/or functionality. For example, many GFCIs now include self-test circuitry, arc fault prevention circuits, miswire protection circuits, and various types of indicator circuitry. Moreover, consumers want wiring devices that include protective functions in combination with non-protective functions. In particular, customers want protective devices such as GFCIs, AFCIs, TVSSs, and the like, in combination with one or more electric service devices such as switches, outlet receptacles, various types of sensors, dimmers, night lights, and etc. Briefly stated, customers want more features in a smaller volume.
One consequence of this development is that relatively high voltage components are brought closer to small signal voltage components on a single printed circuit board. As a result, electrical wiring device designers must now confront the effects of “surface-tracking.” Surface tracking refers to a failure mode wherein defects and contaminants on, and in, the PCB surface form an undesirable circuit path that causes electricity and electrical signals to flow where they are not wanted. When electricity is allowed to track across the surface of a PCB, low voltage circuits may become short circuited and fail. In addition, if unwanted electrical currents cause a device to over-heat, a fire could result.
Referring back to the customer's desire for more features and functionality, many protective devices are now being equipped with microprocessors to meet the aforementioned needs. As a result, the high circuit density noted above may result in AC voltage circuits being disposed next to, or proximate, a processor or other such low voltage circuits. Accordingly, these delicate low voltage signal circuits may be subject to surface-tracking, cross-talk and/or voltage surges. A 6 kV lightning surge, for example, could easily destroy a microprocessor or GFI detector chip.
Another phenomenon that can impact the performance of small signal devices is cross talk. Cross-talk refers to capacitive or electro-magnetic coupling, and may also be the result of stray RF signals or from surface-tracking noise. Cross-talk can be an issue because it may affect the calibration of sensitive electrical circuits. GFCI detectors, e.g., are calibrated to trip when a ground fault leakage current is 6 mA or greater. It is imperative that the detector can obtain a true reading on fault current readings because ground faults can result in human fatalities. With respect to the calibration issue, one must keep in mind that a typical GFI sensor signal is approximately six millionths (0.000006 A) of an Ampere. (A toroidal sensor usually provides the signal input to a GFCI detector). One can see, therefore, how large voltage noise signals can make small signal detections problematic.
Various approaches have been tried to reduce the effects of cross-talk and/or surface-tracking. In one approach, the printed circuit board and all of the circuitry thereon are conformally coated. However, this is a costly manufacturing process and it is subject to manufacturing variabilities. Another approach that has been considered relates to the addition of notches in the PCB. The notches are positioned to isolate the low voltage circuitry from the high voltage circuitry. While this approach can be effective, it also has drawbacks. For example, a relatively large amount of PCB surface area must be devoted to the notches, and as a result, the amount of available surface area for the electronic components can be significantly reduced.
Another issue that is related to device reliability is heat-rise. Devices must operate below a certain temperature or fire may result. One of the main causes of heat-rise are the thermal losses (I2R) caused by resistive interconnections in the AC current path. In other words, the electrical current propagating in the electrical circuit is converted into thermal energy (heat). When a device is packed with a large number of components, it is very practical, from an assembly standpoint, to create a conductive path by interconnecting a number of conductive segments. For example, the practice of routing and interconnecting (hot and neutral) wires through the toroidal assembly results in four interconnections, and four sources of heat-rise.
What is needed therefore is a means for substantially mitigating or obviating the effects of surface tracking, cross-talk and voltage surges without using additional PCB surface area. What is further needed is an effective way to increase the density of electronic components on a PCB while maintaining device reliability. In doing so, it is desirable to limit the number of conductive segments that comprise the AC conductive path from line to load.